Continuous solution polymerization processes generally involve the addition of catalyst to a monomer and solvent mixture. The mixture may be back-mixed giving a uniform polymer in an environment with substantially no concentration gradients. For example, WO 94/00500 describes a solution polymerization using a metallocene catalyst in continuous stirred tank reactors, which may be arranged in series, to make a variety of products.
In the course of the polymerization, typically, a predominant amount (over 50 mol % of the monomer or monomers) is consumed and the polymer formed is dissolved in the solvent. The higher the concentration of the polymer, the higher the viscosity of the polymerization reaction mixture containing the polymer, solvent, and unreacted components. The mixture passes from the polymerization reactor to a finishing section in which polymer, solvent, and unreacted monomer are separated. In the course of finishing, solvent and unreacted monomer are progressively removed from the polymerization mixture until only the polymer remains. The separated solvent and monomer can be recycled back to the polymerization reactor.
The heat of the polymerization reaction can be absorbed by cooling the polymerization mixture fed to the reactor. Alternatively, or in addition, the heat of reaction can be removed by a cooling system, such as by external cooling of the walls of the reactor vessel, or by internally arranged heat exchange surfaces cooled by a heat exchange fluid, or by an external heat exchanger through which the contents of the reactor are continuously circulated, or by an overhead condenser which constantly cools and condenses vapors rising from the reactor, or by a combination of these means.
U.S. Pat. No. 3,912,698 describes a polymerization process that produces a copolymer of ethylene and a C3-C7 alpha-olefin, with the copolymer containing 65-80 wt % ethylene. The process uses a heat exchanger for a liquid recycle stream to permit an increase in reactor capacity while reducing fouling in the context of a multiple flash to remove volatiles.
U.S. Pat. Nos. 6,881,800 and 7,163,989 describe a process and apparatus for the continuous solution polymerization of olefins including ethylene, propylene, and other olefin comonomers. The polymerization reaction takes place under pressure in one or more polymerization reactors, and then the effluent from the reactor or reactors is treated with a catalyst killer and then heated in one or more heat exchangers before being subject to a pressure drop which causes the effluent to phase separate into a polymer-rich phase and a polymer-lean phase. Those phases are separated, with the polymer-lean phase being purified and recycled to be used as solvent. The polymer-rich phase is subject to further separation and purification stages, including passage through a vacuum devolatilizer.
U.S. Pat. No. 8,067,512 provides monomer/solvent separation and recycle processes for continuous supercritical, solution and advanced solution polymerization processes for propylene-containing polymers and their blends. The process includes heating a polymer-lean recycle stream coming from a gravimetric separator positioned downstream of the reactor to form a heated polymer-lean recycle stream, and subsequently combining the heated polymer-lean recycle stream with the one or more homogenous reactor effluent streams to form a heated reactor effluent mixed stream. The heated reactor effluent mixed stream may then be passed through a pressure let-down valve followed and a gravimetric separator, such as to provide for reduced fouling propensity of the polymer-rich phase and sufficient heating of the polymer-rich phase to be able to pass through the remainder of the processing steps to form a propylene based polymer product without additional heat.
PCT Publication No. WO 2011/087728 discloses a plant for the continuous solution polymerization of one or more monomers in a solvent, e.g., a hydrocarbon solvent. The plant comprises a high pressure pump and at least one heat exchanger downstream of the pump. A feed is cooled by three heat exchangers which are refrigerated by means of a common three stage compressor. A method of defouling a heat exchanger is provided in which the level of liquid refrigerant in the heat exchanger is temporarily lowered.
Additional background references include U.S. Pat. No. 7,650,930, PCT Publication Nos. WO 92/14766, and WO 2013/137962.
As discussed above, in continuous solution polymerization processes, the reactor effluent is typically subjected to one or more separation steps in which solvent, residual monomer and other volatiles such as hydrogen are separated from the polymer solution. Generally, the solvent and monomer are recycled, with some heavy components, such as the catalyst solvent (e.g., toluene) and some light end contaminants, such as hydrogen, being permanently removed. The recycled solvent stream which is cooled prior to entering the reactor may contain a trace amount of polymer. However, even this trace amount of polymer can deposit on and consequently foul the solvent driers, heat exchangers, and other equipment through which the recycled solvent passes. This fouling can cause problems with equipment operability that ultimately affect overall process stability. Additionally, during the production of propylene-based polymers, additional fouling can occur when the recycled solvent stream is cooled to a point below the cloud point of the solution, as the propylene-based polymer can begin to precipitate out of the solution and cause fouling. Therefore, there remains a need for improved polymerization processes and apparatuses which reduce or mitigate the effects of such fouling during the production of propylene-based polymers.